1. Field of the Invention
The present invention relates to an integrated chemical reactor for the production of hydrogen from hydrocarbon fuels such as natural gas, propane, liquefied petroleum gas, alcohols, naphtha and other hydrocarbon fuels and having a unique unitized, multifunctional structure. The integrated reactor offers significant advantages such as lower heat loss, lower parts count, lower thermal mass, and greater safety than the many separate components employed in conventional systems to achieve the same end. The integrated reactor is especially well-suited to applications where less than 15,000 standard cubic feet per hour of hydrogen are required.
The present invention also relates to the generation of hydrogen for use in industrial applications, as a chemical feedstock, or as a fuel for stationary or mobile power plants.
2. Discussion of the Background
Hydrogen production from natural gas, propane, liquefied petroleum gas (LPG), alcohols, naphtha and other hydrocarbon fuels is an important industrial activity. Typical industrial applications include feedstock for ammonia synthesis and other chemical processes, in the metals processing industry, for semiconductor manufacture and in other industrial applications, petroleum desulfurization, and hydrogen production for the merchant gas market. The demand for low-cost hydrogen at a smaller scale than produced by traditional industrial hydrogen generators has created a market for small-scale hydrogen production apparatus ( less than 15,000 standard cubic feet per hour (scfh)). This demand has been augmented by the growing enthusiasm for hydrogen as a fuel for stationary and mobile powerplants, especially those employing electrochemical fuel cells, which require hydrogen as a fuel.
Hydrogen is typically produced from hydrocarbon fuels industrially via chemical reforming using combinations of steam reforming and partial oxidation. This is typically achieved at scales larger than one ton per day using well-known process and catalyst designs. For several reasons, it is difficult to adapt these large-scale technologies to economically produce hydrogen at small scales. Typical industrial applications produce far more than 15,000 standard cubic feet per hour (xcx9c1 ton per day), and often employ catalytic steam reforming of light hydrocarbons in radiantly-fired furnaces. Steam reforming of hydrocarbons is illustrated for the simple case of methane below.
CH4+H2Oxe2x86x92CO+3H2
The above reaction is highly endothermic, and the reacting fluid must have energy transferred to it for the reaction to proceed. Further, the extent of the reaction is low at low temperatures, such that greatly elevated temperatures, often as high as 800xc2x0 C., are required by conventional systems to convert an acceptable amount of hydrocarbon to hydrogen and carbon monoxide. The catalyst employed in industrial reactors is typically composed of an active nickel metal component supported on a ceramic support.
The radiantly-fired furnaces employed in large-scale industrial reactors have many disadvantages that make them unsuitable for small-scale systems. The most important disadvantage is the very high temperature of the radiant burners and the gas contacting the reactor surfaces, which are usually tubular in form. The temperature of the radiant burners often approaches or exceeds the melting temperature of the alloy from which the tubes are fabricated. Melting of the tubes is prevented by the rapid endothermic catalytic reaction inside the tubes. If, however, the catalyst fails due to carbon formation, sulfur poisoning or other causes, then the tubes form what is referred to in the literature as a xe2x80x9chot spot,xe2x80x9d which greatly accelerates the failure of the reactor tube in question. In large-scale systems, careful monitoring and control of the furnace and tube temperatures as well as exceptionally rugged construction of the tubes makes the risks of hot spots acceptable. For systems producing below 1 ton per day, however, the complexity and cost of such safety measures can become prohibitive. Nonetheless, small-scale steam reformers utilizing radiant heat transfer are known and described, for example, in U.S. Pat. No. 5,484,577 to Buswell. et al. The extreme measures necessary to control the temperature in arrays of reformer tubes are likewise documented in U.S. Pat. No. 5,470,360 to Sederquist.
A means of transferring the necessary heat to the reacting gases without radiant heat transfer and its attendant risks, which is especially well-suited to small-scale steam reforming, is the use of compact heat exchange surfaces, such as arrays of tubes or finned-plates. The heat transfer mechanism in such devices is dominated by convection and conduction with minimal radiant transfer. An example of this approach is described in U.S. Pat. No. 5,733,347 to Lesuir, wherein finned plates are employed to increase heat transfer. Tubular compact heat exchangers for steam reforming are sold by Haldor Topsoe, Inc. of Houston, Tex.
Conventional hydrogen generation systems employing steam reforming of hydrocarbon fuels typically include three main reaction steps for producing hydrogen; steam reforming, high-temperature water gas shift, and low temperature water gas shift. The important reactions for methane are as follows:
CH4+H2Oxe2x86x92CO+3H2 steam reforming
CO+H2Oxe2x86x92CO2+H2 water gas shift
It is evident from the equation for steam reforming of hydrocarbon fuel that the principal products are hydrogen and carbon monoxide. The carbon monoxide may be converted into additional hydrogen via a catalytic reaction with steam (water gas shift reaction.
The water gas shift reaction is mildly exothermic and thus is thermodynamically favored at lower temperatures. However, the kinetics of the reaction are superior at higher temperatures. Thus, it is common practice to first cool the reformate product from the steam reformer in a heat exchanger to a temperature between 350xc2x0 C. and 500xc2x0 C. and conduct the reaction over a catalyst composed of finely divided oxides of iron and chromium formed into tablets. The resulting reformate gas is then cooled once again to a temperature between 200xc2x0 C. and 250xc2x0 C. and reacted over a catalyst based upon mixed oxides of copper and nickel. An example of this approach is given in U.S. Pat. No. 5,360,679 to Buswell. et al. In cases where an exceptionally pure hydrogen product is required, the temperature of the low-temperature shift converter is controlled by including a heat exchanger in the reactor itself, and an example of this approach is given in U.S. Pat. No. 5,464,606 to Buswell. et al. In all cases, the low temperature shift converter is quite large because of the poor catalyst activity at low temperatures.
In conventional systems, subsets of the process components are connected to one another via external plumbing; each component of the process being typically referred to as a xe2x80x9cunit process,xe2x80x9d in the chemical engineering literature. This approach is preferred in large, industrial units because standard hardware may be used. Owing to the large size of industrial units, the unit process approach also makes shipping of the components to the site of the installation feasible, as combinations of the components are sometimes too large to be transported by road or rail.
For systems producing less than 1 ton per day, however, the unit process approach has many disadvantages. The first disadvantage is the high proportion of the total system mass dedicated to the hardware and plumbing of the separate components. This high mass increases startup time, material cost, and system total mass, which is undesirable for mobile applications such as powerplants for vehicles.
Another disadvantage of the unit process approach in small systems is the complexity of the plumbing system to connect the components. The complexity increases the likelihood of leaks in the final system, which presents a safety hazard, and also significantly increases the cost of the assembly process itself. Moreover, the requirement that each component have its own inlet and outlet provisions also adds considerable manufacturing cost to the components themselves.
A third disadvantage is the high surface area of the plumbing relative to the unit process hardware itself, which means that a disproportionately large amount of heat is lost through the connecting plumbing in small scale systems. This can drastically reduce the thermal efficiency of the system and adds cost and complexity associated with adequately insulating the plumbing system.
A fourth disadvantage to the unit process approach in small-scale systems is that this approach requires a large volume to package, as each component and its associated plumbing must be accessible for assembly and maintenance purposes. This is particularly disadvantageous in space-sensitive applications such as building fuel cell power stations, fuel cell vehicle refueling stations, and fuel cell mobile powerplant hydrogen generation.
Hydrogen is typically separated from the other reaction products using pressure swing adsorption (PSA) technology. The design of these PSA systems is largely dictated by the catalyst chemistry employed in the steam reformer and the low-temperature water gas shift reactor. These catalysts, typically based on nickel metal in the former and copper in the latter case, are extremely sensitive to poisoning and deactivation by sulfur or molecular oxygen. Thus, the incoming feed gas must be carefully treated to remove these materials. Further, the system must protect the catalysts against these agents during startup, shut-down, and during intervals when the system is shut down. Especially in the case of molecular oxygen, exposure of the active catalyst can lead to catalyst damage and even create a safety hazard through pyrophoric oxidation of the finely-divided base metal catalysts.
Several steps are necessary in conventional systems to prevent damage to the reforming and Low Temperature Shift (LTS) catalysts.
(1) During operation, the incoming fuel must be treated to remove both sulfur and molecular oxygen. Sulfur in particular is generally reduced below 1 part per million, and more preferably below 100 parts per billion. This is typically achieved through a combination of a partial oxidation to remove oxygen followed by a hydrodesulfurization (HDS) process. Such systems typically require recycle of high-temperature, hydrogen-rich product gas to the inlet through the use of a gas compressor or a fluid ejector as exemplified by U.S. Pat. No. 3,655,448 to Setzer, U.S. Pat. No. 4,976,747 to Szydlowski and Lesieur, and U.S. Pat. No. 5,360,679 to Buswell. et al. Because accurate temperature control is required for the HDS reaction, several heat exchangers as well as active temperature control logic circuits and flow control valves are also required. Provision of these reactors, heat exchangers, valves, as well as sensors and controls adds significantly to the complexity of conventional systems.
(2) Startup of conventional system requires bringing all of the components to near operating temperature, usually while blanketed in inert gas, then carefully initiating the reaction. Before the system is at operating conditions, full removal of sulfur and molecular oxygen is not guaranteed, so the process feed gas must be vented to the atmosphere, wasting fuel, generating air pollution, and creating a potential safety hazard while further increasing system complexity. Because the added components for fuel pretreatment add significant mass to the system, they also extend the warmup time required for hydrogen production. In situations with a variable hydrogen demand, this can create a need for extensive onsite hydrogen storage to supply the hydrogen demand while the system reaches operating conditions.
(3) During shutdown and periods when the conventional system is not operating, the reaction system is typically purged with inert gases under pressure. Alternatively, substantially leak-tight valves must be supplied to prevent ingress of atmospheric air to the unit with the resultant catalyst deactivation/damage.
For large-scale applications the added cost/complexity of the conventional systems does not adversely affect the system economics. When this traditional approach is applied to small-scale systems, however, the relative cost of these added components becomes disproportionately large, and the resulting hydrogen cost is dominated by the cost of the system. Accordingly, it is not advantageous to simply scale down large scale systems if a small scale system is desired.
Conventional steam reformer systems for natural gas and other light hydrocarbons fall into two broad classes. In the first, the reactors are operated at or near ambient pressure at low temperatures (typically less than 650xc2x0 C.). This is typical of conventional systems designed for small-scale applications producing impure hydrogen. For pure hydrogen to be produced, the reformer product must be compressed to high pressure for subsequent cleanup via PSA, metal separation membranes, or other conventional techniques. Because steam reforming creates additional moles of gas, the compression of the product gas is very energy-intensive and requires expensive and complicated compression and intercooling equipment. The second class of reformers is typically used in large-scale applications and is operated at high pressures (often above 20 bar). Because of the thermodynamics of the steam reforming reaction, these high pressure reactors must be operated at much higher temperatures, often approaching 900xc2x0 C., to attain adequate conversion of the hydrocarbon fuel to hydrogen. The higher temperatures and pressures require the use of more expensive materials of construction than are employed in the low-pressure systems, but this is more than offset by the reduction in reactor volume obtained due to enhanced chemical reaction rates. Unfortunately, in small-scale systems, the provision of compression and pumping equipment to deliver the reactants into a high-pressure (20 bar or higher) reactor can undesirably increase the cost of such a system.
Conventional pressurized steam reformer systems often are operated with very high temperatures in the combustion products used to heat the endothermic reaction zone. This high temperature allows a reduction in the amount of heat transfer area required to complete the reaction, and thus a reduction in reformer cost. Often, the mode of heat transfer to the wall of the tubes in the conventional reformers is a combination of radiation and convection, with the combustion carried out in a conventional premixed or diffusion-flame burner. The operation of the primary steam reformer with such high gas temperatures can lead to significant excursions in the reformer tube wall temperature due either to poor control of the distribution of the hot gases or to poisoning of the reforming catalyst. If the catalyst for the endothermic steam reforming reaction is locally-poisoned, the heat flux from the combustion products to the wall can form a local xe2x80x9chot spot.xe2x80x9d In either case, the increase in the reformer wall temperature can lead to premature reformer structural failure, presenting both a safety and an operational liability.
Conventional systems for hydrogen generation through steam reforming of hydrocarbons have several inherent deficiencies which make them ill-suited to economical small-scale hydrogen production. The first is the requirement for strict control of sulfur and molecular oxygen concentrations in the steam reforming and LTS reactors. The second concerns the problems with operation in the ambient pressure regime where the large volume of reformate gas must subsequently be compressed prior to purification. The third is associated with operating the reactor in the high-pressure regime typical of large-scale units where appropriate compression and pumping equipment adds considerable cost at small scales. The final shortcoming is the risk of overheating the steam reforming reactor structure due to the very high gas temperatures employed in the combustors in conventional systems and their reliance on radiant heat transfer, especially in high-pressure systems as employed in large-scale applications.
It has been recognized previously that integrating the elements of the unit process more closely beneficially reduces heat losses and improves compactness. U.S. Pat. No. 5,516,344 to Corrigan describes a steam reforming system wherein the unit process elements are integrated into a common mounting rack having a reduced requirement for insulation and having improved compactness. This approach, however, undesirably retains the multiple connections and extensive plumbing characteristic of the unit process approach. Moreover, because of its complicated packaging, the assembly of the Corrigan system undesirably presents a significant challenge.
Another attempt at improving compactness is described in U.S. Pat. No. 5,733,347 to Lesieur, wherein the primary reforming reactor and the catalytic burner are integrated into a planar reactor with compact heat transfer surfaces. This reactor requires separate heat exchangers to cool the gas after the primary reformer, as well as separate reactors for the water gas shift. These all require interconnections, as do the array of planar reactors envisioned by Lesieur. These connections once again present the same drawbacks found in unit process reactor systems.
Accordingly, one object of the present invention is to provide a reactor for hydrogen production that avoids the problems associated with conventional systems.
Another object of the present invention is to provide a reactor for hydrogen production that is suitable for applications where less than 15,000 standard cubic feet per hour of hydrogen are required.
Another object of the present invention is to provide a reactor for hydrogen production that is safer and more cost efficient than conventional systems.
Another object of the present invention is to provide a reactor for hydrogen production that is less complex and is more space-sensitive than conventional systems.
Another object of the present invention is to provide for the production of hydrogen from a hydrocarbon fuel such as natural gas, propane, naphtha, or other hydrocarbons low in sulfur content ( less than 100 ppm sulfur by mass).
Another object of the present invention is to produce hydrogen which is substantially pure ( greater than 99.99%) by separating impurities using a pressure swing adsorption (PSA) system.
Another object of the present invention is the provide for the elimination of the pretreatment of the fuel feed to the steam reformer for the removal of sulfur and molecular oxygen.
Another object of the present invention is the provide for the operation of the system in a mesobaric regime, between 4 and 18 atmospheres, where appropriate fluid compression devices of small capacity, low cost, high efficiency and high reliability are readily available, and the resultant thermal efficiency of the hydrogen production system is very high.
Another object of the present invention is to provide for the feedback control of the delivery of fuel and/or air to a catalytic combustor in proportions such that the peak temperature of the gases entering the primary steam reformer does not exceed a safe maximum temperature determined by the metallurgy of the steam reformer.
Another object of the present invention is to provide for the operation of a steam reforming system without a low temperature water gas shift reactor.
Another object of the present invention is to provide for the operation of a hydrogen production system with feedback control of product carbon monoxide content.
Another object of the present invention is to provide a process having a simplified system construction, operation, and control resulting in low cost and relatively fast start-up and shut-down.
These and other objects have been achieved by the present invention, the first embodiment of which provides a reactor, which includes:
a unitary shell assembly having an inlet and an outlet;
a flow path extending within the shell assembly from the inlet to the outlet, the flow path having a steam reformer section with a first catalyst and a water gas shift reactor section with a second catalyst, the steam reformer section being located upstream of the water gas shift reactor section;
a heating section within the shell assembly and configured to heat the steam reformer section; and
a cooling section within the shell assembly and configured to cool the water gas shift reactor section.
Another embodiment of the present invention provides a reactor for the production of hydrogen from at least one selected from the group including natural gas, propane, liquefied petroleum gas, alcohols, naphtha, hydrocarbon fuels and mixtures thereof, the reactor including:
a unitary shell assembly having an inlet and an outlet;
a flow path extending within the shell assembly from the inlet to the outlet, the flow path including a convectively-heated catalytic steam reformer and a convectively-cooled water gas shift reactor.
Another embodiment of the present invention provides a method for producing hydrogen, which includes:
feeding at least one fuel selected from the group including natural gas, propane, liquefied petroleum gas, alcohols, naphtha, hydrocarbon fuels and mixtures thereof, into a reactor which includes a unitary shell assembly having an inlet and an outlet, and a flow path extending within the shell assembly from the inlet to the outlet, the flow path including a convectively-heated catalytic steam reformer and a convectively-cooled water gas shift reactor, whereby hydrogen is produced.
Another embodiment of the present invention provides a method for producing hydrogen from at least one fuel selected from the group including hydrocarbon fuel, natural gas, propane, naphtha, hydrocarbons with  less than 100 ppm sulfur by mass, and mixtures thereof, which includes:
producing hydrogen by steam reforming the fuel; and
substantially purifying said hydrogen with a pressure swing adsorption (PSA) system;
wherein prior to the producing, no pretreatment of the fuel to remove at least one impurity selected from the group including sulfur and molecular oxygen and mixtures thereof is carried out.